1. Field of the Invention
This invention relates to a beam index color cathode ray tube, and, more particular, to a beam index color cathode ray tube provided with an improved electron gun.
2. Description of the Prior Art
In general, the screen which is provided on the inner surface of the face plate of an index color cathode ray tube consists of a fluorescent screen in which phosphor stripes and light absorbing stripes are alternately arranged, an aluminium layer which is adhered over this fluorescent screen, and index stripes which are disposed practically parallel with the phosphor stripes on this aluminium layer.
Since such an index color cathode ray tube does not have the color selecting electrode of the shadow mask color cathode ray tube, the width, in the direction of the phosphor stripes array, of the electron beam which scans the fluorescent screen must be less than the sum of the width of a single phosphor stripe and the width of the light absorbing stripe. However, in practice, the width of this electron beam must be less than 1/3 of the width of a group of phosphor stripes of three colors (i.e. the phosphor trio pitch). For example, since the phosphor trio pitch of a 20-inch cathode ray tube is about 0.9 mm, the width of this electron beam must be less than 0.3 mm.
So as to satisfy the above described dimensional relation between the width of the phosphor stripe and that of electron beam, an electron gun, which comprises a triode portion having a vertically elongated opening and a main lens portion having a circular opening, is employed for this type of color cathode ray tube. The triode portion comprises a cathode, a first grid electrode positioned close to the cathode and a second grid electrode. The main lens portion comprises a third and fourth grid electrode which works as a focusing lens. But, even if the triode portion has a vertically elongated opening, it is not enough to obtain such a narrow electron beam due to a limitation of diameter of the circular opening of the main lens, because in an ordinary electron gun, the diameter of the circular opening of the main lens portion is not so large that spherical aberration of the focusing lense can be neglected due to the space factors in the neck portion, such as, limitation of the inner diameter of the neck portion, size of the supporter fixing the electrodes constructing the electron gun and so on. So, the electron beam can not be correctly focused at the phosphor screen because of the spherical aberration of the focusing lens, and a color cathode ray tube having good color reproduction is not obtained.